Maximizing the percentage of suspended cells recovered live following freezing, archival storage, and thawing requires that a specific temperature reduction profile be applied (see U.S. Patent Application Pub. No. 2011/0308271 and PCT Pub. No. 2011/159934, each of which is incorporated herein by reference). When freezing multiple vials concurrently, it is therefore critical that each vial experience the same desired profile. While passive cell freezing systems offer a significant cost savings over actively managed temperature reduction systems, current passive freezing system designs that offer vial freezing rate uniformity depend upon a circular array of the vials to produce uniform thermal energy release to the surrounding environment. The need for a circular array places a practical limit on the number of vials that can be contained in a freezing unit, as the circumference of the device increases in a linear proportion to the number of vials contained therein.
The sample vials may be confined in a densely packed array; however, this arrangement will require that, for a portion of the vials, the path of heat transfer will at least partially be directed through or past adjacent vials, thereby imposing a variable thermal gradient environment for the vial collection during the freezing process. This condition will inevitably result in a spectrum of temperature reduction profiles, only a fraction of which may be appropriate for optimal cell viability upon thawing. In addition, as the number of vials in the cluster is increased, so will the thermal mass of the collection. To maintain the desired temperature reduction profile, a greater quantity of thermal energy will need to be released per unit time, thereby necessitating a reduction in the thickness of the insulation of the passive cooling device, thereby imposing an even greater imbalance in the freezing rates between the exterior and interior vials of the cluster.
As the sample capacity of the freezing device is increased, a secondary problem arises in that, upon completion of the freezing process, the samples will need to be transferred to archival storage. Transferring the vials individually is both time consuming and presents opportunity for frozen specimens to warm, thereby presenting a great threat to the integrity and viability of the frozen cells.
Thus, there remains a need for passive cooling/freezing devices to achieve the desired temperature reduction profile for clustered sample vials. The present invention meets this need.